It is known that drag created on a body moving through fluid, such as an aircraft moving through air, can be reduced by delaying (i.e., moving leeward or aft with respect to the body) the region on the body where the fluid flow undergoes transition from laminar to turbulent flow. Early attempts to reduce drag involved natural laminar flow (NLF), generally involving efforts to produce airfoil surfaces with extremely low surface roughness. NLF approaches have suffered from the difficulty of manufacturing such low-roughness devices as well as contamination of surfaces by impact, debris, insects, etc.
A second general approach to delaying turbulence involves laminar flow control (LFC) in which boundary layer air from the free stream is drawn through the surface of the body into the interior. In this application, the mass rate of flow across the surface of the body must remain within a desired range. Either too low a flow rate or too high a flow rate can frustrate the goal of delaying the turbulent transition point or even result in an increase in drag. In a typical LFC application, air is drawn through the wing surface of an aircraft by a suction system. This approach, in the past, proved difficult to implement. In order to provide the desired delay of laminar-to-turbulent transition, the boundary layer air must be drawn in with as uniform a distribution as possible. The flow rate through the surface, however, is largely determined (other factors being equal) by the air pressure on the external surface of the body. In many surfaces of interest, such as the wing surface of an aircraft, the external pressure may vary significantly over the surface, causing locally excessive flow where pressures are high and excessively low flow (or even outflow) where pressures are low. External pressure distribution may be particularly variable over the surface in cases where an engine is mounted on the o wing, due to the flow disturbance of the nacelle and strut.
In order to draw fluid through the surface of the body, three main approaches have been attempted in the past: surface slots, porous surface materials, and perforated surfaces. Previous work using each of these approaches has encountered difficulties. Slots present structural problems since it is difficult to transmit shear in the skin of a wing or body which is interrupted by continuous slots. Slots also present difficult fabrication problems. In particular, it has been difficult to provide slots such that the two edges of the slot remain parallel and smooth, particularly when the slots require reinforcement to overcome the structural difficulties.
Porous skin materials also present structural problems since they typically are not capable of withstanding high stresses. Further, porous materials typically have an unacceptably high surface roughness and are prone to permitting outflow of air through the surface in response to local low pressure. Since porous material contains channels that are natural, (i.e., inherent to the material) rather than fabricated under controlled conditions, it is difficult to obtain material with consistent porosity and difficult to predict or model the flow through the porous material (since the exact porous structure may not be known).
Perforated surfaces (substantially non-porous material in which openings have been fabricated) can avoid some of the problems regarding predictability found in porous materials. However, the perforations in many previous perforated materials occupied so much of the area of the material (e.g., more than 10%) that the material was weakened and reinforcement was provided to compensate for the weakening. Many previous perforated materials had perforations that were sufficiently large and spaced sufficiently far apart that a large aerodynamic roughness resulted when fluid was drawn through the perforations. The apparent roughness presented to the flow over the surface is affected by the presence of stream tubes extending above the surface. When these stream tubes are spaced relatively far apart, and accommodate a relatively large flow, the apparent roughness is relatively large. Perforated materials are also subject to blockage from inspiration of debris or from icing. Inspiration of debris is a greater problem for perforations that have larger openings at the external surface. Relatively large perforations also make it difficult to obtain sufficient pressure drop to maintain the desired suction.
Many previous perforated surfaces used in connection with laminar flow control provided perforations which were of equal diameter and equal spatial distribution throughout the perforated region. If this configuration is to be used in an application where external air pressure is non-uniform over the surface, some procedure must be used to achieve uniform mass flow despite the pressure variations. One approach is to provide a suction system which delivers different amounts of suction to different portions of the surface. While this approach can be used to provide a few regions of different suction, the variety and the distribution of external pressure in a practical application such as an aircraft wing generates complex problems in controlling the suction when suction control is the sole means of controlling flow. Another approach is described in U.S. Pat. No. 2,843,341 in which uniformly perforated plates are used in series with a porous material. This approach, however, suffers from many of the disadvantages noted above of porous materials in general. In the context of a system which provides a porous material and a perforated material in series, it has been suggested that the number and diameters of the perforations, can be varied, e.g., in U.S. Pat. No. 2,742,247. However, use of such a perforated material in series with a porous material presents the problems noted above of porous materials in general.